The MRI apparatus is an apparatus for measuring an NMR signal generated by atomic nucleus spins constituting an examinee, particularly a tissue of a human body, and two-dimensionally or three-dimensionally imaging the shapes or functions of a head portion, an abdominal part, four limbs, etc. of the examinee. In an imaging operation, the NMR signal is added with a phase encode which is different in accordance with a gradient magnetic field and also subjected to frequency encode, so that the NMR signal is measured as time-series data. The measured NMR signal is subjected to two-dimensional or three-dimensional Fourier transform to be reconstructed to an image.
In the MRI apparatus, a radio frequency (RF) magnetic field is irradiated to an imaging site of an examinee to obtain an NMR signal. This radio frequency heats the examinee and increases the body temperature of the trunk portion. From the viewpoint of safety, an upper limit value of an SAR value as energy which is absorbed per unit mass of a living body is determined and limited by IEC standards (IEC 60601-2-33: Particular requirements for the safety of magnetic resonance equipment for medical diagnosis) or the like.
The resonance frequency of the radio frequency magnetic field irradiated to the examinee by the MRI apparatus is proportional to the magnetostatic field intensity of the MRI apparatus. Accordingly, in the high magnetic field MRI apparatus, the limitation of the SAR value is particularly severe, and it is controlled by using a method such as optimization of sequence parameters, adjustment of RF to be irradiated, application of SAR reducing sequence while monitored on a real-time basis so that the SAR value of the examinee does not go over the regulation of IEC (see Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, Patent Document 7, for example).